The validation would generate the correct errors to satisfy tests but
would skip clears.

TEST=conformance2/reading/format-r11f-g11f-b10f.html
TEST=conformance2/reading/read-pixels-from-fbo-test.html
TEST=conformance2/rendering/clearbuffer-sub-source.html
TEST=deqp/functional/gles3/fbocolorbuffer/clear.html
TEST=deqp/functional/gles3/fboinvalidate/sub.html
TEST=deqp/functional/gles3/framebufferblit/default_framebuffer_*.html

BUG=angleproject:1954

Change-Id: I0f220d06f98b630be5d27c7ffb1837d4ef29e0ac
Reviewed-on: https://chromium-review.googlesource.com/558786Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>